U.S. patent application number 10/406030 was filed with the patent office on 2003-11-13 for multiple image photolithography system and method.
Invention is credited to Smayling, Michael C., Tittel, Frank, Wilson, William L. JR..
Application Number | 20030210384 10/406030 |
Document ID | / |
Family ID | 29401811 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030210384 |
Kind Code |
A1 |
Smayling, Michael C. ; et
al. |
November 13, 2003 |
Multiple image photolithography system and method
Abstract
A multiple image photolithography system includes a radiation
source (18) projecting electromagnetic radiation along a path. A
reticle cartridge (26) is located in the path of the projected
radiation. The cartridge (26) includes a photomask (34,36) located
in the path of the projected radiation and a Fabry-Perot
interferometer (54) located in the path of the projected radiation.
A radiation-sensitive material (30) is located in the path of the
projected radiation such that the projected radiation encounters
the reticle cartridge (26) before the projected radiation
encounters the radiation-sensitive material (30).
Inventors: |
Smayling, Michael C.;
(Sunnyvale, CA) ; Tittel, Frank; (Houston, TX)
; Wilson, William L. JR.; (Houston, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
29401811 |
Appl. No.: |
10/406030 |
Filed: |
April 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10406030 |
Apr 2, 2003 |
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09692685 |
Oct 19, 2000 |
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6567153 |
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Current U.S.
Class: |
355/53 ; 355/20;
355/75; 430/30; 430/5 |
Current CPC
Class: |
G03F 7/70283 20130101;
G03F 7/70325 20130101; G03F 7/70333 20130101 |
Class at
Publication: |
355/53 ; 355/75;
430/5; 355/20; 430/30 |
International
Class: |
G03B 027/42 |
Claims
What is claimed is:
1. A multiple image photolithography system, comprising: a
radiation source projecting electromagnetic radiation along a path;
a reticle cartridge located in the path of the projected radiation,
the cartridge comprising a photomask located in the path of the
projected radiation and a Fabry-Perot interferometer located in the
path of the projected radiation; and a radiation-sensitive material
located in the path of the projected radiation such that the
projected radiation encounters the reticle cartridge before the
projected radiation encounters the radiation-sensitive
material.
2. The system of claim 1, wherein the photomask and Fabry-Perot
interferometer are located such that the projected radiation
encounters the photomask before the projected radiation encounters
the Fabry-Perot interferometer.
3. The system of claim 1, further comprising: a wafer having a
surface facing the projected radiation, the wafer made, at least in
part, of a semiconductor, and wherein the radiation-sensitive
material is located on the surface of the wafer.
4. The system of claim 1, wherein the photomask comprises: a
transparent plate having a first side and a second side; and a
chrome coating on the second side of the transparent plate, the
chrome coating etched with a pattern and the first side of the
transparent plate facing the projected radiation.
5. The system of claim 1, wherein the Fabry-Perot interferometer is
mounted on a transparent plate.
6. The system of claim 5, wherein the transparent plate is a quartz
plate.
7. The system of claim 1, wherein the Fabry-Perot interferometer
comprises a transmitting layer located between first and second
partially reflecting layers and the first and second partially
reflecting layers have equal reflectivity.
8. The system of claim 7, wherein the reflectivity of the first and
second partially reflecting layers is greater than twenty-five
percent.
9. The system of claim 7, wherein the entire transmitting layer is
located between the first and second partially reflecting
layers.
10. The system of claim 1, further comprising: a lens assembly
located between the reticle cartridge and the radiation sensitive
material in the path of the projected radiation, the lens assembly
focussing at least one image of the photomask onto the radiation
sensitive surface.
11. The system of claim 1, wherein the Fabry-Perot interferometer
is a component of an interferometer plate, the interferometer plate
also comprising a transparent plate, and the reticle cartridge
further comprises: an interferometer frame located outside the path
of the projected radiation and coupled to both the interferometer
plate and the photomask, the space between the interferometer plate
and the photomask being sealed by the interferometer frame.
12. A reticle cartridge for providing multiple mask images
comprising: a photomask having first and second sides, the second
side partially coated with a radiation blocking material and having
coated and uncoated portions, the uncoated portions defining a
pattern; an interferometer frame attached to the second side of the
photomask and defining a first area of the second side, the first
area being not in contact with the interferometer frame and within
the portions of the second side that are in contact with the
interferometer frame; and an interferometer plate comprising a
Fabry-Perot interferometer, the interferometer plate coupled to the
interferometer frame and covering the first area of the second side
of the photomask, the combination of photomask, frame and
interferometer adapted to be removably inserted into a stepper.
13. The reticle cartridge of claim 12, wherein the Fabry-Perot
interferometer comprises three layers, the first and third layers
made of partially reflective material having equal reflectivity and
the second layer made of transmitting material.
14. The reticle cartridge of claim 12, wherein the radiation
blocking material is chrome and the pattern defined by the uncoated
portions corresponding to a layer of a circuit design.
15. The reticle cartridge of claim 12, wherein the photomask, the
interferometer frame and the interferometer plate define a sealed
volume.
16. A method for projecting multiple radiation images onto
photoresist, comprising the steps of: inserting a first substrate
covered with photoresist into a stepper; placing the first
substrate within a radiation path projected by a radiation source
of the stepper; inserting a first photomask and a Fabry-Perot
interferometer into the stepper; placing the first photomask and
the Fabry-Perot interferometer within the radiation path with the
first photomask between the radiation source and the Fabry-Perot
interferometer, and the Fabry-Perot interferometer between the
first photomask and the first substrate; projecting radiation from
the radiation source that passes through the first photomask and
the Fabry-Perot interferometer before reaching the photoresist.
17. The method of claim 16 further comprising the step of: removing
the first substrate, the first photomask, and the first Fabry-Perot
interferometer from the radiation path; inserting a second
substrate covered with photoresist into the stepper; inserting a
second photomask into the stepper; placing the second photomask
within the radiation path of the stepper with the second photomask
between the radiation source and the second substrate; projecting
radiation from the radiation source that passes through the second
photomask before reaching the photoresist.
18. The method of claim 17. wherein the steps of inserting a second
substrate and a second photomask into the stepper occur before the
step of placing the first photomask and the Fabry-Perot
interferometer within the radiation path.
19. The method of claim 17, wherein the substrate is made of
semiconductor material.
20. The method of claim 17, wherein the first photomask is attached
to the Fabry-Perot interferometer.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates generally to the field of focussing
electromagnetic radiation, and more particularly to a multiple
image photolithography system and method.
BACKGROUND OF THE INVENTION
[0002] Integrated circuits are produced from wafers of silicon or
some other semiconductor material. A typical process for producing
an integrated circuit includes photolithography. The design of the
circuit to be produced requires that certain surface portions of
the silicon have specific electronic characteristics. Those
characteristics are modified by exposing the silicon to other
elements that migrate into the silicon crystal. Because the change
in conductivity is desired only in certain areas, a material is
used to coat the silicon and impede the migration. Photolithography
is used to remove the coating from only those surface areas where
migration, and the concomitant modification of electronic
characteristics, is desired. Photolithography is also used to place
conductive materials at specific points on the wafer. The
manufacture of liquid crystal devices and magnetic heads may also
include the use of photolithography.
[0003] The smaller the scale at which electronic characteristics
can be manipulated, the more circuit elements can fit onto a chip
of given area. More features may also be included in a liquid
crystal device or magnetic head of a given size as a result of more
exacting photolithography. As the size of circuit elements has
decreased, photolithography equipment has become more exacting so
that greater resolution can be achieved. Replacing a
photolithographic stepper in order to increase the resolution and
the depth of focus is very expensive. Large capital costs delay the
improvement of photolithographic resolution.
[0004] Photolithography may be employed many times in the
manufacture of a single device. For example, an integrated circuit
may be formed of over twenty layers, the pattern for each layer
projected onto the device using photolithography. Over the course
of adding many layers to a device, the surface of the device may
develop undulations. In may be desired that the photolithographic
equipment be able to focus the pattern of light on a surface with
varying vertical dimensions. New generations of photolithographic
steppers are able to focus radiation with greater precision across
the surface of the wafer or other device, i.e., in the horizontal
dimensions. As the precision of focus in the horizontal direction
is increased, the range in the vertical direction over which this
precision occurs in decreased. The range over which the image stays
in focus in the vertical dimension is called the depth of field.
Thus, it may be desired to focus radiation very narrowly in the
surface dimensions and maintain that focus over some range in the
vertical dimension.
SUMMARY OF THE INVENTION
[0005] Accordingly, a need has arisen in the art for an improved
photolithography system. The present invention provides a multiple
image photolithography system and method that substantially reduce
or eliminate problems associated with prior photolithography
systems.
[0006] In accordance with the present invention, a multiple image
photolithography system includes a radiation source. The radiation
source provides electromagnetic radiation that is then projected
along a path. The system also includes a radiation-sensitive
material located in the path of the projected radiation. A reticle
cartridge is located in the path of the projected radiation between
the radiation source and the radiation-sensitive material. The
reticle cartridge contains a photomask and a Fabry-Perot
interferometer. The photomask and interferometer are located in the
photomask so as to lie in the path of the projected radiation.
[0007] More specifically, in accordance with one embodiment of the
present invention, the reticle cartridge is positioned with the
photomask preceding the interferometer in the projected radiation
path.
[0008] Also in accordance with the present invention, a method for
projecting multiple radiation images onto photoresist includes
inserting a substrate with a photoresist coating into a stepper and
positioning the photoresist within the path of radiation projected
by a radiation source in the stepper. A Fabry-Perot interferometer
and a photomask are inserted into the stepper and each is
positioned in the radiation path with the photomask between the
interferometer and the radiation source. Radiation is projected
from the radiation source, passes through the photomask and
interferometer and then reaches the photoresist.
[0009] Technical advantages of the present invention include
improving the depth of focus of the radiation pattern projected by
a stepper onto a radiation-sensitive material. Another technical
advantage is allowing for a Fabry-Perot interferometer to be placed
within and removed from the radiation path of a stepper without
expensive modifications to the stepper. Another technical advantage
is an increase in the pattern resolution without the large capital
cost of upgrading stepper equipment. Other technical advantages of
the present invention will be readily apparent to one skilled in
the art from the following figures, descriptions, and claims.
Individual embodiments of the invention do not necessarily include
all the technical advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of the present invention
and its advantages thereof, reference is now made to the following
description taken in conjunction with the accompanying drawings,
wherein like reference numerals represent like parts, in which:
[0011] FIG. 1 is a perspective view of a stepper;
[0012] FIG. 2 is a radiation path diagram illustrating one
embodiment of the present invention;
[0013] FIG. 3 is a cross-sectional view of a reticle cartridge
illustrating one embodiment of the present invention;
[0014] FIG. 4 is an expanded view of a portion of FIG. 3;
[0015] FIG. 5 is a cutaway view of a reticle cartridge illustrating
one embodiment of the present invention;
[0016] FIG. 6 is a radiation path diagram illustrating multiple
images in one embodiment of the present invention; and
[0017] FIG. 7 is a separate and combined view of the intensity of
multiple images.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The preferred embodiments of the present invention and its
advantages are best understood by referring now in more detail to
the figures in which like numerals refer to like parts. FIG. 1 is a
perspective view of a stepper 10. The stepper 10 is a very
expensive apparatus that projects a pattern of electromagnetic
radiation onto a radiation-sensitive material.
[0019] For example, integrated circuits may be formed on wafers.
The wafers are composed of a silicon crystal having impurities and,
in most situations, dopants. The wafers may also be composed of
another semiconductor crystal such as gallium-arsenide. A chemical
reaction forms a surface layer that protects the crystal. For
example, oxygen may react with the silicon to form a layer of
silicon dioxide that covers the surface of the silicon crystal. A
radiation-sensitive material, sometimes called photoresist, is
coated over the protective layer. The wafer, with its protective
layer and photoresist coating, is inserted into the stepper 10.
[0020] In the stepper 10, a pattern of radiation can then be
projected onto the photoresist. The physical characteristics of the
photoresist are affected by exposure to radiation. For one type of
photoresist, positive resist, exposure to electromagnetic radiation
and then a chemical process results in the removal of the
photoresist, leaving in place the photoresist that was not exposed
to radiation. For another type of photoresist, negative resist,
exposure to electromagnetic radiation enables the photoresist to
remain during the subsequent chemical process so that only the
unexposed photoresist is removed. In either case, once portions of
the photoresist are removed, a chemical reaction can occur to
remove the protective layer, for example, silicon dioxide, only
where the photoresist has already been removed. Once specific areas
of the wafer have been exposed to the semiconductor crystal, the
characteristics of those areas can be specifically modified. For
example, dopants or electrical contacts can be added. The process
can be repeated as many times as necessary to add all of the
features desired by the integrated circuit designer. Some
integrated circuits require tens of layers.
[0021] The scale of the modifications that can be made to the
structure of an integrated circuit depends to some extent on the
scale of radiation that can be focused on the photoresist in the
stepper 10. The photoresist coating of the wafer is not always
perfectly flat. For example, previous processing may have developed
undulations in the surface. It is important that the focused
radiation be extended sufficiently in the vertical dimension to
encounter the photoresist regardless of the undulations. The extent
to which the radiation extends in the vertical dimension is called
depth of field or DOF. The requirements that the radiation be
precisely focused in the horizontal dimensions but extend in the
vertical dimensions can be difficult to satisfy simultaneously.
[0022] The stepper 10 includes a first portion 12 through which
wafers are inserted. The stepper 10 positions the wafers to receive
electromagnetic radiation. The vertical and horizontal positioning
of the wafer is performed to a very small scale. A smaller depth of
field requires greater accuracy in the vertical placement of the
wafer. Providing such accuracy in a stepper can be very expensive.
A second portion 14 of the stepper 10 contains the radiation path
and optical components. The optical components are also placed with
great precision. The path is not necessarily a straight line
because mirrors and other optical components can be used to direct
the radiation. In one embodiment, the radiation source projects
radiation upward from the bottom of the second portion 14. The
radiation encounters several optical components before being
redirected downward to a wafer positioned to receive it. The
stepper 10 also includes a third portion 16 through which
photomasks are inserted into the radiation path. The third portion
16 of the stepper 10 is configured to receive the photomask in a
structure having a specific size and shape. A reticle cartridge is
a photomask container having the size and shape necessary to be
inserted into the stepper 10. As with the wafers, the positioning
of the reticle cartridge is performed by the stepper 10 on a small
scale.
[0023] FIG. 2 is a radiation path diagram illustrating one
embodiment of the present invention. Each of the components shown
in FIG. 2 is located in the stepper 10. A radiation source 18 is
configured to project electromagnetic radiation through an exit
aperture 20. The radiation source can be a laser or some other
device capable of providing radiation having a tightly bounded
range of frequencies. The radiation then passes through a series of
lenses. For example, in FIG. 2, lenses 22',22",22'" are positioned
in the path of the projected radiation to modify the optical
characteristics of the radiation. For example, the lenses
22',22",22'" are used to collimate the light into a wide parallel
beam, for uniform illumination of the mask. A mirror 24 redirects
the radiation path. The mirror and lenses 22',22",22'" are mounted
in the stepper in precisely determined locations and are generally
difficult to modify or remove without damaging the ability of the
stepper 10 to precisely focus radiation patterns on
radiation-sensitive materials.
[0024] The reticle cartridge 26, in contrast, can be removed from
the radiation path without damaging the stepper 10. The reticle
cartridge 26 contains the photomask which provides the pattern that
the radiation reaching the radiation-sensitive material will take.
Each layer used in the manufacturing process of an integrated
circuit can require a different pattern and, consequently, a
different reticle cartridge 26. The lenses 22',22",22'" can be used
to direct the radiation to the photomask. The radiation that is
able to traverse the reticle cartridge 26 forms a pattern. That
patterned radiation is focused by lenses 28',28" making up a
projection lens system 28. The pattern can be focused to finer
horizontal dimensions at the radiation-sensitive layer 30 than at
the reticle cartridge 26. For instance, in many steppers, the
projection lens will reduce the image size by a factor of 5. The
silicon substrate 32 supports the radiation-sensitive layer 30.
[0025] FIG. 3 is a cross-sectional view of the reticle cartridge 26
illustrating one embodiment of the present invention. The reticle
cartridge 26 includes a transparent plate 34. The transparent plate
can be made of many materials including soda lime, borosilicate
glass, and fused silica. A thin layer of etched chrome 36 is placed
on one surface of the transparent plate 34. The etchings of that
layer 36 determine the radiation pattern that will traverse the
reticle cartridge 26 to reach the photoresist 30. The etched chrome
layer 36 may cover only a portion of the surface of the transparent
plate 34. The transparent plate 34 and etched chrome layer 36
together embody a photomask.
[0026] The edges of the etched chrome layer 36 are attached to an
interferometer frame 40 by adhesive 38. The adhesive 38 can be
double back tape, glue or another adhesive. The frame can be
coupled to the edges of the layer 36 in a sealed manner. The frame
is also attached with adhesive 38 to an interferometer plate 42.
That attachment can be sealed so that the chrome layer 36,
interferometer frame 40, and interferometer plate 42 define a
sealed volume. The sealed volume prevents impurities from reaching
the focus plane at the chrome layer 36. The adhesive 38 can be
replaced with a mechanical attachment. A portion 44 of the
interferometer plate 42 is identified.
[0027] While the interferometer plate 42 and photomask are attached
to form a reticle cartridge 26 in one embodiment, another
embodiment could separate the interferometer plate 42 and the
photomask. A stepper 10 could be provided that contained openings
into the radiation path for both a reticle cartridge 26 containing
the photomask and a separate cartridge containing the
interferometer plate 42.
[0028] FIG. 4 is an expanded view of a portion 44 of the
interferometer plate 42. The interferometer plate 42 includes a
transparent plate 46, a first partially reflective deposition layer
48, a transmitting deposition layer 50, and a second partially
reflective deposition layer 52. The deposition layers together form
the Fabry-Perot interferometer 54. The first and second partially
reflective deposition layers 48,52 can be reflective dielectric
stacks. The transmitting deposition layer 50 can be a layer of
silicon dioxide. The reflectivity of the first 48 and second 52
partially reflective layers can be set close to equal. The
characteristics of the Fabry-Perot interferometer 54 are then
determined by that reflectivity and the distance between the
partially reflective layers. Many embodiments of the present
invention will employ Fabry-Perot interferometers in which the
partially reflecting layers will have reflectivities above
twenty-five percent. In one embodiment the reflectivity is
ninety-five percent. In another embodiment of the invention, the
Fabry-Perot interferometer 54 can be formed by enclosed a radiation
transmitting gas between layers of partially reflective
material.
[0029] FIG. 5 is a cutaway view of the reticle cartridge 26
illustrating one embodiment of the present invention. The
transparent plate 34 and etched chrome layer 36 of the photomask
are shown. The interferometer frame 40 is also shown. The
orientation is FIG. 5 is vertically opposite the orientation of
FIGS. 2, 3, and 4. A region 58 of the chrome layer 36 is surrounded
by the interferometer frame 40. The region 58 is in the path of the
radiation projected by the radiation source 18.
[0030] FIG. 6 is a radiation path diagram illustrating multiple
images in one embodiment of the present invention. Radiation
received at the pre-mask lenses 22 (including lenses 22',22",22'")
is collimated to illuminate the etched chrome layer 36 that forms a
pattern in the radiation. The patterned radiation then reaches the
Fabry-Perot interferometer 54. Some of the radiation traverses the
interferometer 54 without deviation, while some is reflected within
the interferometer 54 and exits at a deviant point. Some radiation
intensity is lost in the interferometer 54. The reticle cartridge
26 includes both the etched chrome layer 36 and the interferometer
54. A projection lens system 28 focuses both the deviated and
undeviated radiation where the photoresist 30 would receive the
radiation. The radiation focuses at multiple points 56 in the
radiation path because of the Fabry-Perot interferometer 54
deviations. Each point in the path comprises an image of the
pattern from the etched chrome layer 36.
[0031] FIG. 7 shows the intensity of the multiple images in both a
separate and a combined view. The top chart shows the intensity at
points along the vertical axis of each image produced. The largest
image, formed by the undeviated radiation, is positioned where the
single image would be positioned if the reticle cartridge 26 did
not include the Fabry-Perot interferometer 54. The depth of focus
(DOF) is shown for that image. The lower chart shows the total
intensity at points along the vertical axis resulting from the sum
of the images produced by the interferometer 54. The increased DOF
is also shown.
[0032] In some applications DOF is less important than horizontal
resolution. The Fabry-Perot interferometer 54 can improve DOF and
horizontal resolution to different degrees depending upon the
characteristics of the interferometer 54. For example, the
reflectivity of the partially reflective layers 48,52 or the
distance between those layers can be modified to change the DOF or
horizontal resolution characteristics of the resulting image. One
method of calculating the DOF and resolution effects of including
an interferometer 54 in the reticle cartridge 26 is discussed in an
August 1999 article by Erdelyi et al. in the Journal of the Optical
Society of America A. Optics and Image Science entitled "Simulation
of coherent multiple imaging by means of pupil-plane filtering in
optical microlithography" hereby incorporated by reference in its
entirety. Equations can translate the characteristics of the
Fabry-Perot interferometer 54 into characteristics of a pupil-plane
filter. Software is available, such as Prolith/2 from FINLE
Technologies Inc., that will simulate the DOF and horizontal
resolution resulting from the pupil-plane filter
characteristics.
[0033] Although the present invention has been described with
several embodiments, various changes and modifications may be
suggested to one skilled in the art. It is intended that the
present invention encompass such changes and modifications as fall
within the scope of the appended claims.
* * * * *